Analytical balance



12 Sheets-Sheet l M. BOZOIAN ANALYTICAI.. BALANCE June 1, 1954 Filed Jan. 26, 194e June 1, 1954 M. BozolAN ANALYTICAL BALANCE Filed Jan. 26, 1948 l2 vSheets-Sheet 2` IN V EN TOR.

June 1, 1954 M BOZOlAN 2,680,012

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June l, 1954 M. BozolAN ANALYTICAL BALANCE 12 Sheets-Sheet l0 Filed Jan. 26, 1948 .M EZ Ww y MM m l MW June 1, 1954 M. BozolAN ANALYTICAL BALANCE 12 Sheets-Sheet 11 Filed Jan. 26, 1948 l 1N VEN TOR. /I//c//AH 902 o/A A/ 14% VMM June l, 1954 M. BozolAN ANALYTICAL BALANCE 12 sheets-sheet 12 Filed Jan. 26 1948 HIIIIIHHIGIIIHIHIIIIHIIH s mm M N/ i. m Mawr A my M MW NN x Patented June 1, 1954 UNITED STATES TENT OFFICE ANALYTICAL BALANCE Application January Z6, 1948, Serial No. 4,220

34 Claims. l

This invention relates generally to weighing machines and refers more particularly to improvements in analytical beam type balances.

An object of this invention is to provide a balance rendering it possible to weigh unknown loads automatically in less time and with greater precision than heretofore thought possible with orthodox manually operable precision balances. With a balance embodying the features of 'this invention, it is only necessary for the operator to place the unknown load upon the balance pan and to manipulate a suitable starting control. Immediately upon manipulation of the starting control, a series of operations are automatically effected in their proper sequence and the weight of the unknown load on the beam is directly indicated by an indicator. Thus the chance of "nun man error is reduced to a minimum, and the balance may be operated to weigh unknown loads with extreme accuracy.

./i more detailed object of this invention is to provide a displacement detector operable to translate deflection of the beam from gravitational equilibrium into an electric signal having a inagnitude proportional to displacement of the beam and having a polarity dependent upon the direction of movement of the beam about its support.

Another object of this invention is to utilize the inverse feed baci; System for stabilizing the balance by providing a servo motor electromagnetically coupled to opposite ends of the beam in a manner to oppose deflection of the beam. by the unbalanced load on either of the beam pans.

Still another feature of this invention is to magnify the electrical signal produced by the detector with a servo amplier having the input side electrically connected to the detector through an anti-hunt circuit and having the output side electrically connected to the servo motor through a range selector circuit. The anti-hunt circuit electrically anticipates and minimizes hunting of the servo systems by modifying the dynamic character of the input signal into the servo amplifier by a leading phase shift circuit, and thereby reduces destructive mechanical vibration of the balance beam to a negligible value.

The so-called range selector circuit embodies a plurality of resistors fixed and adjustable,

which are connected in series and/or parallel between the output of the amplifier and the input of the servo motor. The resistors correspond to diiferent Weighing ranges, and it is a further object of this invention to consecutively connect the resistors in circuit With the servo motor in accordance with the voltage at the output side of the amplifier so that the opposing force applied to the beam is a function of the unknown load on the beam.

A further object of this invention is to provide circuit means between the output of the range selector circuit and the input of the servo motor enabling compensating for the sensitivity error. This error is a direct function of the load on the balance beam, and becomes apparent as a decrease in beam deflection for a given weight. It is the function of the compensating circuit to integrate the incoming signals in a manner such that the sensitivity error is completely compensated for.

A still further object of this invention is to connect an electrically operated meter in the output circuit of the range selector circuit calibrated in weight units and operable to indicate an unknown weight on the balance beam.

Another feature of this invention is to provide a servo counter circuit energized by the output voltage of the amplifier and embodying a member movable step by step throughout a predetermined path of travel. By virtue of the above general circuit arrangement, the amplifier voltage output is directly proportional to the unknown load for each weighing range and the distance the member moves is, therefore, dependent upon the unknown load.

Still another object of this invention is to provide Weight applying mechanism operable by the member to selectively apply different weights to the beam depending upon the extent of movement of the member or upon the output voltage of the amplifier.

A further feature of this invention is to couple the weight applying mechanism to a number wheel indicator which functions to integrate the numerical value of the weights used to establish gravitational equilibrium of the beam. This wheel may be used in conjunction with the meter noted above to display the exact weight of the unknown load on the beam as will be described more in detail below.

A still further feature of this invention is to provide an arrangement responsive to operation of the number wheel to correct beam errors and to incorporate additional means operated by the weight applying means to compensate errors in the individual weights. Thus unequal lengths of the beam arms and inaccuracies of the actual weights do not affect the accuracy of the balance. This is important in that it enables obtaining precision balance operation with weights formed within practical tolerances, and eliminates the necessity of frequent checks on either` the weights or balance beam.

The foregoing as well as other objects will be made more apparent as this description proceeds, especially when considered in connection with the accompanying drawings, wherein:

Figure l is a block diagram indicating a number of the units employed to produce automatic operation of the balance;

Figure 2 is an electrical diagram of certain parts of the balance control equipment;

Figure 3 is a circuit diagram of the range selector;

Figure i is a circuit diagram of the power supply unit;

Figure 5 is a circuit diagram of the servo counter;

Figure 6 is a diagrammatic perspective view oi the weight operating mechanism;

Figure '7 is a diagrammatic perspective view of the weight loading mechanism;

Figure 8 is a diagrammatic elevational view of the weight indicating wheel assembly;

Figure 9 is a diagrammatic perspective view of the program apparatus and showing a part of the range selector in conjunction with the program apparatus;

Figures l0, 1l, 11A and l2 are elevational views of the weight actuating cams;

Figure 13 is a diagrammatic view showing one arrangement for correcting errors resulting from beam inaccuracies;

Figure 14 is a diagrammatic View showing an arangement enabling inaccuracies in the weights to be compensated for;

Figure l5 is a circuit diagram showing a modied form of control for the balance;

Figure 16 is a diagrammatic perspective view of a modified form of weight loading apparatus.

This invention is shown for the purpose of illustration as used in connection with weighing machines of the type commonly known as analytical balances. This general type of balance is indica-ted in Figure l of the drawings by the box 20 and comprises a number of parts which form no part of this invention. In the interests of simplicity, therefore, only those parts of the balance required for illustration purposes are shown and these parts are merely diagrammatically illustrated. Referring to Figure 2 of the drawings the numeral 2l designates a beam pivoted intermediate the ends on a bearing 22 and having load supporting pans 23 and 2li suspended from opposite ends of the beam.

Displacement detector The block 25 in Figure l of the drawings indicates a displacement detector which translates deflection of the beam 2l caused by the application of an unknown load on one of the pans into an electrical signal. Referring to Figure 2 of the drawings, the detector embodies a capacitance bridge comprising xed plates 2t and movable plates 2. The plates 2l' are respectively secured to opposite ends of the beam 2l and the plates 26 are xedly supported in any suitable manner directly opposite the movable plates 2l. The iiXed plates 26 or" the bridge are respectively connected to one of the ends of suitable coils 28 symmetrically arranged with respect to an intermediate coil 29 and inductively coupled to the latter by an air path designated by the numeral 29. The opposite ends of the coil 29 are electrically connected to a conventional type of electronic oscillator designated generally by the numeral 3U and operable to supply alternating current into the capacitance bridge. Briey the oscillator embodies an electron tube 3| connected in circuit to the primary coil of a transformer 32 having the secondary coil connected to the coil 29.

Connected in series with the coils 28 are a pair of resistors 33 and eac-h resistor is shunted by rectifier elements 34. The rectifier elements produce equal and opposite direct current voltages across the resistors 33 when the balance beam is at gravitational equilibrium. The coils 2S are also respectively connected to output terminals 35 and 36, which in turn, are electrically connected to the input circuit of a suitable servo amplifier about to be described. In order to enable electrically connecting the terminals 35 and St directly to the grids of electron tubes in the input circuit of the servo amplier, provision is made for applying a direct current bias voltage to the capacitance bridge. This direct current voltage is supplied to a third terminal 3'! by a power pack; also to be presently described.

lt will be understood from the above that the electronic oscillator Si] or the source of alternating energy is inductively connected through the coils 23 to two legs of the capacitance bridge, and the juncture of these legs is coupled to the terminal 3l, which is shown in Figure i or" the drawings as also connected to ground through the lower leg til of the voltage divider. rihe other two legs of the bridge are formed by the beam 2l which is capacitatively connected to ground by the inherent stray capacitance of the beam. This is the equivalent of grounding fulcrum 22 and enables employing accepted balance construction, wherein the ulcrum is formed of agate, sapphire or some equivalent wear resisting material other than metal. Such materials possess high electrical insulation qualities, and consequently the ulcrum or orthodox balances cannot be directly connected to ground by an electrical conductor. The beam of the orthodoi; balances is formed of meta-l and ccordingly may be connected to ground by an electrical conductor. However in precise analytical balances such practice is prohibitive, because the conductor, regardless of its size, tends to destroy the sensitivity of the balance, and renders the balance useless in the milligram range, for example.

it follows from the foregoing that the displacement detector is composed of two simiiar circuits, each having a capacitor (25 and coils 2B and a resistance 33 shunted by a half-wave rectiner Sli. Each capacitor has a stationary plate and a movable plate 2l carried by the beam 2 i. The coils 2S are inductively coupled to the alternating current source 3d by the coil 29. The return path for each branch of the displacement detector is electrically complete because the junction between the two resistors 33, shunted by the rectifiers 3d, and the junction between the moving plates of the capacitors are connected together (as far as the alternating current is concerned). This is true because the junction between the resistors 33 is returned to ground via the terminal 3l, and the junction between the moving plates of the capacitor is returned to ground by virtue of the stray capacitance of the beam 2l to ground. The output terminals oi the detector are connected across the opposite ends of the two resistors 33. The rectiers 3s, shunted across the resistors 33, are connected back to back in series opposing relationship across the terminals. Thus, when the beam 2l is in equilibrium, equal and opposite direct current voltage is developed across the resistors 33 and the output is zero. On the other hand, when the beam 2i is not in equilibrium the impedance of one capacitor is reduced While the impedance of the other capacitor is increased. In consequence of this impedance change, the rectified voltage drops across the two resistors 33 are not equal and a direct current output signal results with a magnitude dependent upon the extent of beam movement and a polarity dependent upon the clockwise or counterclockwise travel of the beam from equilibrium.

It follows from the foregoing that minute displacements of the balance beam 2i increase the capacitance of one capacitor and decrease the capacitance of the other capacitor. When the beam 2i is in equilibrium, the direct current voltage developed across the terminals 35 and 3l exactly counterbalance the direct current voltage across the terminals 37 and t6. Minute displacements of the beam 2| generate direct current voltages of negative or positive polarity between the terminals 35 and 3,5, depending upon the direction in which the beam is displaced. Thus the output of the displacement detector, across the terminals 35 and 36 is a direct current voltage proportional to the displacement of the beam and having a polarity depending upon the direction of displacement.

Anti-hunt network The block 38 in Figure 1 of the drawings indicates a corrective circuit which is referred to herein as an anti-hunt network. It will be understood as this description proceeds that the operation, stability and accuracy of the automatic balance is made possible by a number of closed servo loops. It is characteristic of sensitive and high speed electro-mechanical servo sys* tems to exhibit a phenomenon known as hunting. This co-called hunting, if not prevented, manifests itself as a destructive mechanical Vibration of the balance beam and its associated system. it is the purpose of the anti-hunt circuit to anticipate and check hunting by modifying the dynamic character of the input signal to the servo amplier, to be described presently. As shown in Figure 2 of the drawings, resistances 4@ are respectively connected to the output terminals 35 and 36. These resistances are shunted by capacitive elements ii and are connected to the output terminals 42 and 43 of the anti-hunt circuit. Capacitors ll in conjunction with potential divider resistors IMJ and 45 provide a leading phase shift circuit for the incoming signals.

Servo amplifier The box 4t in Figure 1 of the drawings indicates an amplifier having the input side electrically connected to the output of the anti-hunt circuit 3B for magnifying the electrical signal to sufficient voltage and power levels to operate the power consuming components of the succeeding units of the automatic balance. Referring again to Figure 2 of the drawings, the output terminals l2 and t3 of the anti-hunt circuit 3S are respectively connected to the grids of a pair of voltage amplifier triodes 5U electrically connested in push-pull. This push-pull stage provides in a conventional manner across resistors 5i and 52 magnified voltage replica of the signal. This magnined voltage or signal is in turn applied directly to the grids of the triodes 53 and E@ connected to provide a push-pull cathode follower, power output stage. The lower output impedance of this type of stage provides the inherent electrical properties necessary for satisfactory damping of the servornotor to be presently described.

Power supply rhe power supply is more or less conventional and is indicated in Figure 1 of the drawings by the box 55. rihis supply circuit shown in Figure c comprises generally an electrical transformer rectifier tube 51, filter 58 and potential divi ing resistors 59 and 60. The resistors 5S and it are connected across the ground and terminal Si. This terminal is connected to the resistors 5l and 52 in the ampliiier, and is also connected to the plates of the amplifiers 53 and t. The arrangement is such that a relatively high, direct current voltage is available across the terminal El and ground suiiicient in magnitude to energize the various electronic circuits incorporated in the balance control system. The adjacent ends of thc resistors Eil and @D are connected to the terminal 3l with the result that a relatively small direct current Voltage is available across the terminal 31 and ground which is suiiicient to polarize the capacitance bridge in the detector circuit. The various windings Wi, W2, W3 and the leads Ll and L2 are provided to supply alternating current power where needed.

Range selector it is neither economical nor practical to design the components of the automatic balance to possess an accuracy greater than 1 to 3 per cent. However, in analytical balance work, it is frequently necessary to weigh unknown loads up to 2cd grams to within 1/20 milligram, or othe words, to one part in four million. The technique used in the automatic balance to ach ve this degree oi over-all accuracy is to divide the weighing operation into seven decade ranges as follows: (l) the kilogram range, 2) the 1Q() gram range, (3) the 10 gram range, (4) the l gram range, (5) the 100 milligram range, (6) the lo milligram range, and (7) the 1 miiligrarn range.

The accuracy of the automatic weighing in any one oi the above ranges is designed for practical reasons to be within l to 3 per cent. The Weighing procedure starts with the kilogram range and progresses automatically through to 'he one milligram range. Thus any error in measure-ment that may occur in a preceding range multiplied ten times in the succeeding range, thereby facilitating economical instrumentation for detecting and correcting or ccmpensating for such error.

This range selecting function is the duty of the circuit arrangement indicated in Figure l oi the drawing by the block S2. As also shown in Figure 2 of the drawings, the output of the servo ampliner di! is connected to the input sideof the range selecting circuit which includes a seven position (l kg. through 1 mg. inclusive) switch having three decks indicated by the numerals t3, ti and t5. As more clearly shown in Figure 9 of the drawings, each deck of the switch comprises a rotatable contact t5 and seven stationary contacts indicated by the indicia 1 kg., 1GO g., 10 g., l g., 10G mg., 10 mg. and 1 mg. syn.- metrically disposed about the common axis of rotation of the respective contacts 66 for successive engagement by the latter. The three contacts (it are connected together for rotation as a unit and are operated by program apparatus to be later described in detail. The switch decks E53, and Gli successively introduce into the range selector' output terminals 6l and ES series and series parallel iixed and adjustable resistors. rI'he series resistors comprise items t9, 'ill and l l the series parallel resistors comprise the pairs 'l2 and i8; and lil; and 'lil and 8i). The adjustable resistors 63 to 'll inclusive and i8 to 8@ inclusive are used as calibration means for each individual range. Furthermore, capacitances 'i5 to 'Vl inclusive used in parallel to resistors l i to l2 respectively, and the calibration resistors i8 to lil inclusive used in parallel to the output circuit promote critically damped response of the balance.

The resistors till to ifi inclusive are successively connected in series with the output terminals el and t28 by the three deck switch. Additional resistors lil, 79 and 3B of the adjustable type are in shunt with the output terminals lll and @3, and are respectively connected to the resistors l2, lil and 'if-l. By means oi these controls, each range may be calibrated so that the servo amplifier output is, for example, 1GO volts for the iull load of that range. As noted above this calibration is achieved by the series of adjustable resistors Eil, "le and 'di for the relatively heavy load ranges of l iig., 1GO g. and l0 g. respectively, and by series-parallel resistors (l2 to 'lll and "it to El?, respectively) ior the smaller load ranges of l g., 100 mg., 10 mg. and l mg. The reasons for the series-parallel adjustable resistor type o control are two in number: (l) to avoid extremely high ohmic resistors which are expensive and not as stable as wire wound controls; and (2) to take advantage of the parallel component of the control as additional damping on the servomotor.

The function of the switch decl: is to introduce either multiplier resistor Si or 82 into an indicator circuit 33. The resistor Si is ten times greater, ohmically, than the resistor 82 and is adapted to be successively connected in the indicator circuit by the switch deck 65 for each decade range noted above except the one milligram range. When this final range is connected to the output of the servo amplifier, the switch deck 35 connects the resistor 82 in the indicator f circuit. rlhis arrangement increases the sensitivity of the indicator circuit ten times and enables using the greater power of the ten milligram range for the one milligram range. In other words the resistors l and 80 for the ten milligram range may also be used when the three switch decks are in their iinal positions at the one milligram range.

Meter indicator The block Bxl in Figure l of the drawings designates a meter. This meter is connected in the circuit and is of the one milliampere type which, in conjunction with the multiplier resistors ril and 32, becomes a 100D ohm/volt voltmeter indicator. The dial scale of the meter is preferably calibrated in gram or milligram units suitable for the range desired to be covered.

S croc motor The block Sil in Figure 1 of the drawing indicates a torque motor rendering it possible to utilize the inverse feed back system of stabilizing the automatic balance. This motor applies a torque or couple to the balance beam 2l which is opposite and slightly less than the torque or couple applied to the beam by the load on the latter. As Will become apparent from the following description, the servomotor applies an opposing torque to the beam 2l Without physical connection to the beam and thereby preserves the sensitive characteristics of the beam for weighing operations.

In detail the servomotor comprises two solenoids 81 and 88 respectively associated with op- Dosite ends of the beam 2l. Each solenoid in turn comprises a permanent bar magnet 89 and a coil lil encircling the bar. The bars are respectively suspended from opposite ends of the beam 2l in any suitable manner with like poles at corresponding ends thereof in order to minimize external magnetic effects. The lower ends of the coils 9i] are electrically connected together and the upper ends of the coils are respectively connected to the output terminals 5l and 68 of the range selector. Thus the application of electrical power to the coils il@ produces a clockwise or counterclockwise torque on the balance beam 2i, depending upon the polarity of the input voltage.

Sensitivity error compensator The unit is designated generally in Figure l of the drawings by the box 9i and comprises the circuit shown more in detail in Figure 2 of the drawings. Even in the nest analytical balances, there are a number of sources ol errors which are difficult, if not prohibitive, as far as cost is concerned to correct or compensate. One such error, identified as the sensitivity error, is a direct function oi the load on the balance beam and becomes apparent as a decrease in beam pointer deflection for a given unit Weight. For example in a typical analytical balance, unloaded, the beam pointer deects ten divisions upon the pointer index when one milligram is placed upon the 4load pan. However, when the balance is loaded with 200 grams per pan, its rated full load, the beam pointer deilects eight divisions from gravitational equilibrium when the same one milligram is placed upon the load pan.

This decrease in sensitivity, for an increased load upon the balance beam, is very dil'lcult to correct or compensate in present day balances, and is the chief reason that direct reading projection type pointer index devices are not popular. It is the function oi block Si, interconnected as shown in Figure 2, to integrate the incoming signals in such a manner that the sensitivity error is compensated for in the output voltage to the servomotor.

In detail the foregoing is accomplished by connecting two resistors 92 and 93 in series across the input to the servomotor 36. The resistor $3 is an adjustable rheostat element with an ohmic value determined by rotation of its control shalt. The resistor 93 is automatically adjusted in proportion to the load on the balance beam 2l, and this is accomplished by operatively connecting the control shaft to the counter or wheel to be more fully hereinafter described for indicating the units of weight loaded on the beam by the weight loading apparatus also to be later described. rThe resistor 92 is placed external to the resistor 93, and is a means for spreading the control range of the latter resistor.

Serco Counter For the purpose of this description, the equipment in blocks 2li, 25, 38, lill, 55, 52, 9i and Sil may be considered the primary servo loop. This 9 arrangement makes it possible for the servo ampliiier 48 to have an output voltage directly proportional to the unknown load on the beam for each decade range. This output voltage is applied to a secondary servo loop comprising the mechanism in blocks 20, 25, 33, d8, 55 (previouslT described), and in blocks 34 and 95 to be presently described. Included in the secondary servo loop is a shaft driven by an electronically controlled motor which is operated by the amplifier output voltage to angularly position the shaft in a discreet, step-Wise manner directly proportional to the units weighed, less one unit, in existing decade range.

The above is accomplished by the servo counter indicated generally by the block 94 in Figure 1 of the drawings and shown in detail in Figure 5 of the drawings. The electronically controlled motor referred to above is indicated in Figure 5 of the drawings by the numeral 95 and the shaft driven by this motor is designated by the reference character el'. The motor 96 is a reversible shaded pole type, and, in addition to driving the shaft Sl, also imparts rotation to a wiping contact arm 98. The free end of the arm S8 is adapted to successively engage sixteen contact points P through PIE which are equally spaced about a circle described by the free end of the arm 9g. The initial contact position or home is identifled by contact point Pil. This contact point is connected to ground by means of two series-connected resistance elements RH and RI 8, Rl l being adjustable for calibration purposes. The contact points are interconnected by equal resistance elements Ris to R33 inclusive having ohmic values equal in turn to RIB.

Contact point Pl5 is connected by lead SQ to a voltage regulated power supply represented by the circuit lil. This may be any suitable electronic regulator, such as the one diagrammed, capable of maintaining the output voltage constant within close limits.

In Figure 5 the voltage regulator circuit Hill obtains its direct-curr;ent power from lead M which terminates in the power supply of Figure fl. Because of the connection t9, the voltage drops between contact points P to Pi, Pl to P2, P2 to P3, etc., are maintained substantially constant and equal to each other and may have a value of say l volts. Hence with the wiper in the home position shown, it is approximately positive l0 volts, direct-current, above ground.

The wiper is connected to one position Y! of a double-pole triple-throw switch lill, |02 by means of a lead m3. The normal position of the poles of this switch are as shown in Figure with pole itil engaging position Y! and lii engaging position Y2.

The poles oi the double-pole triple-throw switch are joined by the terminal leads of the capacitor lli identified further as a memory capacitor. In the Iii-X2 position, the capacitor l il is charged by voltage from the output side of the amplifier through resistor R34. In the position Zl-ZFE, the capacitor iii'e is discharged into resistor R35.

From terminal Y2 the circuit leads to a resistance-capacitance time-delay and ltering network composed of resistor R36 and capacitor lill. The purpose of this network is to minimize spurious transients and power-supply-frequency signais from actuating the control grid of the thyratron tube m8.

The thyratron tube |08 is normally conducting, powered by the alternating current supply provided by transformer I9 whose primary side is excited by a convenience outlet via leads LI and L2. The conductive current of Hi8 provides a negative voltage drop across the resistor R37 and capacitor ISS, and is applied through resistor R33 to the control grid of the thyratron tube l it rendering it normally non-conductive. The tube Ht is energized by means of a transformer lli having its primary leads connected to one of the shaded-pole windings W5 of the electronically controlled motor 95.

The reversible shaded-pole motor has its primary winding W4 energized from the power line via leads LI--L2- The two shaded-pole windings WE and W6 are normally open and should either winding be individually closed, the motor rotor will rotate in a clockwise or counter-clockwise direction depending upon which winding is closed. In the automatic balance, winding W5 is electronically closed when desired by making the thyratron tube Il conducting. This condition effects rotation of the motor rotor and the coupled driven shait Sl in a clockwise direction.

In order to move the wiper to the home position, the winding W6 of the motor is energized by a switch H2 forming a part of the program apparatus to be presently described. This allows rotation of the motor rotor in a counter-clockwise direction until cam H3 opens a switch H4 included in the circuit of winding WB. In the actual construction the cam H3 is operated by the shaft 9! in a manner such that the switch l I4 is opened when the wiper 9E reaches the contact point Pil or home position. When the shaded-pole windings W5 and W6 of the motor 9d are both open, the motor rotor, as Well as the shaft 91, is dynamically braked due to the energized motor ield winding W4. y

The principle of operation of the servo counter is as follows: With the automatic balance in any one of its decade ranges the double-pole triplethrow switch lill, 102 is momentarily operated (by the program apparatus to be presently described) to the position III-X2 where the condenser l is charged substantially to the servo amplifier output for that range. The resistor R311 with the capacitor |06 insures that the charged level is an average servo output and not a spurious or transient peak due to momentary shock or vibration to the automatic Ibalance. Upon completion of this operation the switch l lll, m2 is again operated to return the switch poles to the Yl-YZ position shown in Figure 5 of the drawings wherein the charged capacitor is placed in the grid circuit of the thyratron tube Hi8. The negative polarity of the charge on capacitor |96 tends to extinguish conduction of the thyratron tube 105. I-Iowever, since the position Y! is also in the grid circuit and is approximately positive 10 volts above ground, the thyratron tube IDB will not extinguish unless the charge on the capacitor IUS is greater than l0 volts. By adjusting the potential of Pil by the control resistor RI l', the voltage on the capacitor |06 which will just extinguish the tube Il can be predetermined. The control resistor Ril is also used to compensate for small manufacturing differences in the thyratron tube HB5.

In the automatic lbalance it is desirable that the servo-counter always counts or rotates one position less than the actual units of weights on the balance for that range. The reason for under-counting or under guessing of the load is that it is more economical to correct for such errors in the guess by adding weights than by removing the same. In other Words the mechanism of the automatic ybalance is designed to be a forward, additive process rather than an additive and subtractive or reversible process.

Since in any decade range each unit of weight may represent 10 volts and since the position PE! is approximately 10 volts, the thyratron tube Edil cannot be extinguished unless the condenser charge is greater than 10 volts. It will be recalled that this condenser potential is directly proportional to the servo amplifier output and hence proportional to the load on the balance. As a specific example: if the automatic balance is operated on the grams decade range and there are S grams on the vbalance beam, the servo ampliner output will be 80 volts. This 80 volt level can be memorized by capacitor lii by first moving the switch lili, m2 into position Xi-XZ and by subsequently returning the switch to position "Yi-Y2, or normal. rhis places into the grid circuit of tube GBS a net bias of negative I volts with respect to the cathode of tube le@ which is the reference and ground. Ten volts are subtracted, because Pil is approximately positive 10 volts above ground, Since the tube |93 has its grid biased to negative 70 volts, the tube ceases conducting current, and the bias voltage produced across the resistor R31 and condenser iilii disappears.

As a result of the above, the tube H0 conducts energizing the winding W through the transformer i i I. When the win-ding W5 of the motor 9B is energized, the shaft 9i rotates in a clockwise direction causing the wiper 98 to successively engage contact points PI, P2, etc. When the wiper 98 engages the contact point PI, for example, the net grid bias for the tube |03 is reduced volts and is negative 60 volts. Moreover, each engagement of the wiper with the successive contact points reduces the net voltage by 10 volt increments. Finally when the wiper 9S engages the contact Pl the net grid bias is zero volts and the tube 98 is rendered conductive, again biasing the tube ll to the non-conductive state. When the tube ll is non-conductive the motor 98 is dynamically braked to a quick stop because the field winding W4 is continuously energized.

It will be noted from the foregoing that the electronic circuit of Figure 5 is in itself a servo i system positioning the wiper and hence the shaft Si so that a contact point (P0 through PI 5) is selecte-d having a potential approximately equal to the charged potential level on the memory capacitor lii. The purpose of resistor R39 is to provide a conductive ypath to ground for the control grid circuit of the tube 168 during that period of time that the wiper travels from one contact point to the next. This high resistance conductive path permits stable operation of the servo counter in its hunt for the proper angular position necessary to balance out the memory condenser voltage.

There is still to explain the gating action of the servo counter, the go or no-go feature which provides economical and positive counting of the unit weights, less one. As described above the contact point P6 is approximately l0 volts above ground potential and any weight less than a unit weight on any range will be unable to extinguish the thyratron tube HM, Thus the motor 96 is prevented from rotating the shaft 91 and wiper 9.?. It will also be noted that one unit weight places a potential of 10 volts on the memory condenser |06 and, since the contact point Pt Gil is substantially '10 volts, it follows that the tube Hi8 would still conduct current and the net grid circuit voltage is zero.

The control ratio of the tube |03 is such that approximately negative three-quarters to volt net bias applied to the control grid circuit will cause the tube to cease conducting. Thus by adjusting the resistor RII and thus the positive potential of all the Contact points with respect to ground, the unit voltage level necessary to index the wiper can be predetermined. In this application RH is adjusted so that it requires 1% units of weight, plus or minus 0.2 unit before the wiper indexes That is, on the l0 gram decade range, for example, the wiper will not index for 10 grams weight, may index one position for 10.05 grams, and must index one position for any weight above 10.45 grams. This tolerance has been set up to insure a count of one less than the total units weighed in any range. It is necessary to use fifteen units for each decade range to provide complete additive coverage for a bad guess in any preceding range. The gating action described makes possible economical and accurate over-all automatic weighing.

After each servo count operation for any rang the memory capacitor IBB is discharged into resistor R35 by moving the switch to position ZI, Z?. to prepare this capacitor for the next weighing operation.

Weight loading device As will be more fully hereinafter described, the servo counter driven shaft 9T is mechanically coupled to the weight loading device indicated generally in Figure l of the drawings by the reference numeral and shown in detail in Figure 6 of the drawings. Briefly the servo counter shaft 91 is adapted to be selectively connected to one of six counter shafts embodied in the weight loading device, and respectively corresponding to the kilogram decade range, one hundred decade range, ten gram decade range, one gram decade range, one hundred milligram decade range and the ten milligram decade range. Supported on the above shafts are suitable cams for operating pusher bars which in turn serve to automatically load weights on the beam to balance the unknown weight for each decade range,

In detail it has been noted above that the angular position of the servo counter shaft 97 is proportional to the unit Weights measured in any' decade range minus one. Furthermore, because of the tolerance limits of plus or minus 0.2 unit weight used for the go-no go gating action, fifteen standard weights will be necessary for each decade range, except the kilogram range. In order to avoid handling a multiplicity of weights, it is preferred to employ a technique which may be identified as the 1 2-2-5-5 system. This means that for each range, except for the kilogram range, five weights having values of 1, 2, 2, 5 and 5 properly loaded can result in any desired value from one through fteen. The kilogram range is excepted because it is only necessary in this range to determine whether or not a one hundred gram standard weight shall be applied to the beam. Thus in order to cover the kilogram, the hundred gram, the ten gram, the one gram, the 100 milligram and the ten milligram ranges, a total of twenty-six weights are required. These standard or reference weights must be actuated in the proper sequence by the servo counter at the direction of a suitable program apparatus which will be presently described in detail.

In Figure 6 of the drawings, a shaft I |5 is shown and this shaft is coupled in any suitable manner to the servo counter shaft 91 for rotation as a unit with the latter. The shaft M is in turn coupled by pinions i I6 to six shafts I Vi, lit, I It, 20, I2! and |22 in a manner to rotate the latter at the same speed as the servo counter shaft Si. rhese six shafts respectively correspond to the kilogram decade range, 100 gram decade range, l0 gram decade range, one gram decade range, 100 milligram decade range and the milligram range. The shafts II'E to |22 inclusive are connected to their respective driving gears by electromechanical clutches |23, IM, |25, It, |21 and t28 which are selectively operated by the program apparatus to be presently described.

Referring again to Figure 6 of the drawings, it will be noted that a cam |2 is secured to the kilogram shaft ||1 for operating a pusher bar which, in turn, manipulates a ring type reierence weight I3! of 100 grams. As shown in Figure 7 of the drawings, a hanger |32 is suspended from the right hand end of the balance beam 2| and a weight carrier |33 having iingers |34 is supported on the hanger for vertical movement relative thereto. The ring weight |3| is carried by the fingers |34 and is adapted to be supported on fingers |35, projecting outwardly from the hanger, in response to downward movement of the carrier relative to the hanger. It will, of course, be understood that upward movement of the carrier |33 lifts the ring weight |3| off the hanger or balance beam.

Secured on each of the remaining shafts i8 to 522 inclusive are five cams identified by the weight units I, 2, 2', 5 and 5. Each cam is engaged by a pusher bar iA, 2A, ZA, 5A and bA. Thus the weight loading apparatus embodies twentysix pusher bars counting the kilogram bar |30. As stated above the pusher bar |30 applies a weight ring I3 to and removes the same from the hanger |32 on the beam by parts |33 and |35. These parts are duplicated for each pusher bar as indicated in Figure 7 of the drawings, wherein six of the twenty-six weight operating parts are shown. The remaining twenty weight loading parts are identical and need not be shown. It will suiiice to point out that each group of ve weight loading parts corresponds respectively to the luc gram range, the 10 gram range, the 1 gram range. the 100 milligram range and the 10 milligram range. The 1 milligram range is not considered as this unit of weight may be read directly from the meter sii. The speciic contour of the cams is shown in Figures 10, 1l, 11A and 12 of the drawings. The IA cam being shown in Figure l0, the 2A cam being shown in Figure 11, the ZA cam being shown in Figure 11A, the 5A, 52A. cams are combined in actual practice and are shown in Figure 12 of the drawings.

Alternative weight loading apparatus En order to avoid handling a large number of weights, the arrangement shown in Figure 16 may be resorted to. In this construction the standard weights up to 150 milligrams may be applied by a chain weight Itii connected to the right hand end of the balance beam 2| and operated by the counter shaft iid through an auxiliary shaft |31. The torque thus produced upon the balance beam is equivalent to adding physical weights, and eliminates the necessity ot handling the very small weights. The remaining parts of the loading apparatus shown in Figure 16 of the drawings are similar to the parts previously described in connection with Figure 6, and corresponding parts are designated by the same reference numerals.

Number wheel indicator The weight loading apparatus is coupled to a number wheel indicator shown generally in Figure 1 of the drawings by the box |38 and illustrated more in detail in Figure 8 of the drawings. This indicator functions to integrate the numerical value of the standard weights used to establish gravitational equilibrium of the balance beam 2 i. The indicator presents the above weight value in six gures, three before and three after the decimal. The fourth place after the decimal is presented by the reading on the meter 84. Thus the unknown load on the balance beam 2| is evaluated to better than one part in four million for a two hundred gram load.

In detail the indicator comprises six wheels |40, Isl, |42, |43, IM and |45. Each wheel has applied to the periphery thereof the numerals 0, 1, 2, etc., through 9 spa-ced equal distances from each other. The wheels are independently rotatably supported, and are respectively coupled to the shafts to |22 inclusive of the weight loading apparatus shown in Figure 6 of the drawings by any suitable means not shown herein, The arrangement is such that the number wheels are independently positioned at the desired numerals starting with the wheel |510 and Working over to the wheel |45. Should any number wheel index past the numeral 9, the preceding wheel will index one step in accordance with conventional Veeder Root type counters. 1t will, of course, be understood that the usual means is provided on the counter for resetting the wheels to the zero positions when desired. The number wheel indicator is also coupled in any suitable manner to the adjustable resistor S3 in the error compensator circuit 9| for adjusting this resistor in proportion to the load on the balance beam. Any simple mechanical or electrical means may be employed to connect the resistor $3 to the number wheel indicator, and this disclosure is not complicated by a specific illustration of this means.

Compensator for beam and weight errors The circuit for accomplishing the above results are indicated generally by the box |116, and are shown in detail in Figures 13 and 1li of the drawings. Briefly it will be understood that another source of error in an analytical balance is caused by unequal length of the beam arms. Although this error may be as small as one part in 100,000 or less, nevertheless, for loads of 10 or 20 grams, it becomes greater than desired for accurate Weighing. Another source of error that should be corrected in order to obtain precise weighing involves the accuracy of the physical weights employed. In accordance with this invention, the weights used need not be calibrated as precisely as weights employed in conventional analytical balances. In other words, with this invention, the weights used may be manufactured to nominal values within reasonable tolerances, because any error in the specic weights is compensated for by the circuit arrangement to be presently described.

Figure 13 of the drawings features a circuit 50 adapted to correct errors in the beam structure. In detail this circuit comprises a pair of coils iti respectively encircling the magnetic core members 09 associated with the solenoids 87 and 88.

The lower ends of the coils |5| are electrically connected and the upper ends of the coils are connected to the output side of a direct current power pack designated generally by the box |52. Electrically connected in series with the coils is a fixed resistor |53 and an adjustable resistor |55. Inasmuch as the beam error is a function of the integrated weights loaded on the beam, the adjustable resistor shaft may be coupled to the number wheel indicator |352 for action by the latter. The connection between the adjustable resistor |55 and the number wheel indicator |38 is not specifically shown, but may be either` mechanical or electrical, whichever is found more convenient.

The circuit arrangement shown in Figure 14 is provided for correcting errors in the values oi the individual weights and is indicated generally by the numeral |55. This circuit also embodies a pair of coils magnetic cores S9 of the solenoids B1 and 88. The lower ends of the coils |55 are electrically connected together and the upper ends oi the `coils |55 are connected to a power pack |51. Connected in the circuit |55 are a plurality of switches |523 corresponding in number to the number of weights and respectively operated by the pusher bars shown in Figure 6 of the drawings. In order to simplify the disclosure, only nve switches |58 are shown in Figure 14 in operative relation with the pusher bars for one group of weight actuating cams, and it is to be understood that this arrangement is duplicated for each group of cams. Also connected in the circuit |55 in series with the switches adjustable resistors |59 for controlling the now current from the power pack E51 to the coils i523. Thus the circuit |55 is energized each time one of the weights is loaded, and any error in the value of the weight applied is corrected.

Program. apparatus The program apparatus is indicated generally by the numeral |50 in Figure l of the drawings, and is shown more particularly in Figure 9 of the drawings. The purpose of the program apparatus is to operate the various parts of the automatic balance in their proper sequence, and to initiate the operation of the balance as well as provide for resetting the several parts subsequent to operation thereof. It will be understood that the program apparatus is merely diagrammatically shown in Figure 9 of the drawings, and that no effort has been made to show all of the parts of this apparatus. In the interests of simplicity, only sufficient structure is shown to afford an understanding of the function of this apparatus.

ln detail a shaft |B| is shown in Figure 9 of the drawings as operated by an electronically controlled motor |62 similar to the servo counter motor Se in that it is of the shaded-pole type having three windings |63, IM; and |65. The winding iGfi is the field winding and is continuously7 energized from the power line when the balance is in operation. The winding |63 is one of the shaded-pole windings, and is electronically energized to cause rotation of the motor armature or shaft I 8| in a clockwise direction.

The winding E55 is the other shaded pole wind- ,ing, and is energized through mechanical means |56 respectively encircling the f 53 are having` a position |63 (corresponding to the initiate position), and having a position i 59 (corresponding to the reset position). When the switch is the initiate position, |53, a circuit to the thyratron tube |15 is completed, and this tube is rendered conductive. Thus 'the motor wind- |Si is energized, and the shaft |i5| is rotated in a clockwise direction. Rotation of the shaft in a clockwise direction imparts a rotation to the contact wiper |1| relative to the adjacent switch timer plate |12 and angularly moves the wiper |1| from the home position H into successive contact with the contacts K, KI, K2, and etc., corresponding in number to the number of instrumentalities of the balance to be automatically controlled by the program apparatus. The contacts K, Ki, K2, and etc., are wired together and are connected to the grid of the tube ill.

lx/.iovement of the wiper |1| from one Contact to the adjacent contact must be delayed for a period of time suicient to enable the instrumentality of the balance represented by the Contact to complete its task. In this connection it will be noted that contacts D are positioned between the contacts l, KI, K2, and etc. The contacts D are electrically connected together and to a source of negative potential |13 having the positive lead grounded. Through this ground connection the contacts D are electrically connected to the grounded side of a capacitor I1l| having the opposite side connected to a brush |15. The brush bears against an insulated slip ring or sleeve |16 which also forms an anchor for the inner end of the wiper |1I.

It follows from the above that as the wiper i1| leaves Contact h (for example) to move to contact K, it brushes over Contact with the result that the capacitor |13 is charged to a large negative potential. When the wiper |1| arrives at contact K, this negatively charged capacitor is placed directly across the grid-cathode circuit of the tube |10, and the latter is biased to the off position. Thus the motor winding |53 is deenergized and rotation of the motor as well as the shaft is dynamically braked.

Since different functions of the balance require different time intervals, two or more resistors |11 and |18 are provided to control the time intervals. '.in detail the capacitor |14 being negatively charged stops rotation of the shaft ISI and the negative charge discharges through resistor |11 and/or resistor |18. As a result the shaft IGI and associated switch gear complete a new lineup of circuits by rotating to another position. lt will, of course, be understood that if the discharge path of capacitor |14 is through both resistors, the time delay is longer than if the discharge is through only resistor |11. For some functions of the apparatus, only short time delays are required and in such instances the resistor |13 is shorted to ground by a switch gear |19. This switch gear |19 comprises a switch plate lili) and a wiper |3I connected to the shaft |6| for rotation thereby to successively engage suitable contacts |82 on the plate |85. These contacts are electrically connected together and are connected in the relay circuit between the resistors |11 and |18. The wiper |8I is grounded so that each time it engages a contact |82 on the plate |80, the resistor |18 is shorted, hastening discharge of the capacitor |14. Thus conduction of the tube |10 is reestablished and rotation of the shaft IGI to the next position is accomplished. 

